Plain English Meaning
ATC Towers are the staffed facilities at busy airports that talk to pilots by radio and tell them when and where to taxi, take off, and land. They keep aircraft from running into each other on the ground and during takeoff and landing.
Full Definition
Air Traffic Control Towers are FAA facilities located at airports that issue clearances and instructions to aircraft operating in the immediate vicinity of the airport, including on taxiways, runways, and within the surrounding controlled airspace. They are responsible for the safe, orderly, and expeditious flow of traffic on the airport surface and in the air close to the field.